I've had some in the fridge as long as 2 1/2 weeks. And it got used up before it went bad. Hubby and I don't drink much milk.
That is part of the reason I use the powdered milk. I can keep it on the shelf and make up what I need.

Money was tight when I grew up so my mom used to mix 1/2
powdered milk to 1/2 whole milk and we grew up drinking that.
It must have been alot cheaper.
I could write a book about money saving things my parenst did
but we never felt poor. I guess it helped that everyone at school
was about as poor as I was. Rich kids went to their own school.

My mother and my grandmother both only used powdered milk while we were growing up, I didn't have whole milk till I was 6 and in the first grade, spent the whole first week of schooling throwing up and no one knew why. One of the cafeteria ladies was a family friend and starting giving me the 2% till I got used to that. I hope to never have to give that to my kids as their only dairy though I do add it to mac and cheese or mashed potatos to up their nutrient values.

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I ask for so little. Just let me rule you, and you can have everything that you want. Just fear me, love me, do as I say, and I will be your slave. Jareth, Labyrinth